Phosphine coke inhibitors for EDC-VCM furnaces

ABSTRACT

A method of reducing the formation of coke deposits on the heat-transfer surfaces of an ethylene dichloride to vinyl chloride pyrolysis furnace comprising exposing the heat transfer surfaces of said pyrolysis furnace to a phosphine selected from the group consisting of phosphines with the general formula:                    
     wherein R 1 , R 2  and R 3  are selected from the group consisting of hydrogen, chlorine, alkyl, aryl, alkylaryl and arylalkyl, wherein R 1 , R 2  and R 3  can be the same or different.

FIELD OF THE INVENTION

This invention relates to a method of inhibiting coke formation inpyrolysis furnaces. Specifically, this invention relates to a method ofinhibiting coke formation in ethylene dichloride/vinyl chloridepyrolysis furnaces.

BACKGROUND OF THE INVENTION

Thermal pyrolysis or cracking of ethylene dichloride (EDC) to vinylchloride (VC) is the major industrial process for vinyl chloride monomer(VCM) production at present. The thermal pyrolysis process entails theuse of pyrolysis furnaces, also known as EDC-VCM furnaces, to thermallyconvert EDC to VC. The pyrolysis process occurs as a homogeneous,first-order, free-radical chain reaction. The general reaction mechanisminvolves the following steps:

Initiation: ClCH₂CH₂Cl→ClCH₂CH^(2·)+Cl^(·)

Propagation: Cl^(·)+ClCH₂CH₂Cl→^(·)ClCHCH₂Cl+HCl^(·)ClCHCH₂Cl→ClCHCH₂+Cl^(·)

Termination: Cl^(·)+ClCH₂CH_(2·)→ClCHCH₂+HCl

A typical pyrolysis furnace has three sequential building blocks:convection section, radiant section, and transfer line exchanger (TLE).Metal alloy serpentine coils run through the convection section and theradiant sections, and connect to the TLE. The convection sectionutilizes the convection heat from the radiant section to preheat, andsometime to vaporize and preheat EDC feed. The coils in the radiantsection function as the pyrolysis reactor where the preheated EDC feedis cracked to VC.

Because of the severe operation environment in the pyrolysis furnaces,iron alloys of high Ni and Cr content are common materials ofconstruction of the pyrolysis furnaces. The TLE is a heat exchangedevice, which quickly quenches the effluent from the pyrolysis reactor.The quenching is to stop any product degradation under adiabaticcondition in the past furnace zone.

Industrial pyrolysis reactors are typically operated at temperatures offrom about 470° C. to about 550° C. (about 878° F. to 1022° F.), atgauge pressures of from about 1.4 Mpa to about 3.0 Mpa (about 200 psigto about 435 psig) and with a residence time from about 2 seconds toabout 30 seconds. EDC conversion per pass through a pyrolysis furnace isnormally maintained around 50-55% with a selectivity of 96-99% to vinylproduct. VC and HCl are the major components in the pyrolysis reactoreffluent. By-products from the pyrolysis process range from the verylights, such as methane, acetylene, ethylene and methyl chloride, to theheavies, such as carbon tetrachloride, trichloroethane and solidcarbonaceous material. Solid carbonaceous material is usually referredto as coke, and coke is an unwanted by product of the pyrolysis process.

Higher conversion in the pyrolysis process is, in most cases, desired.However, increasing cracking severity beyond conventional operationconditions generally leads to only a small increase in EDC conversion atthe expense of the selectivity to vinyl chloride product. Furthermore,any outstanding increase in cracking severity causes a drastic increasein coke formation and a sharp drop in VC selectivity.

Fouling of the pyrolysis furnace occurs due to formation of coke. Infact, coke formation often becomes the major limitation in pyrolysisfurnace operation and VC production. Formation of coke with resultantfouling decreases the effective cross-sectional area of the process feedflow through the pyrolysis furnace and the TLE, and thus increases thepressure drop across pyrolysis furnaces. In order to compensate for thepressure buildup, generally, a reduction in EDC feed rate is necessary.A reduction in EDC feed rate means an overall reduction in production.Another undesirable feature of coke formation is that the coke is a goodthermal insulator, and thus coke formation reduces the heat transferacross the walls of the pyrolysis reactor. The reduction in heattransfer requires a gradual increase in furnace firing duty to maintainthe cracking reactions at a desired conversion level. Furnace fire dutythus can also become the limiting factor for conversion and overall VCproduction. To maintain the capacity and the fire efficiency ofpyrolysis furnaces at optimum levels, pyrolysis operation has toperiodically cease for coke removal (decoke), which causes productiondown time.

Known methods for the removal of coke from pyrolysis furnaces includecontrolled combustion or mechanical cleaning, or a combination of bothmethods. In the combustion process, a mixture of steam and air ofvarious steam/air ratios is admitted in the pyrolysis furnace at anelevated temperature, and the coke in the reactor is burnt out under acontrolled condition. This process is conventionally referred as hotdecoke. For the mechanical cleaning, coke is physically chipped off thepyrolysis furnace inner surface and removed from the reactor. Bothcracking and the hot decoke operations expose the pyrolysis furnace to acycle between a HCl and chlorinated hydrocarbon-rich reducingenvironment and an oxygen-rich oxidizing environment at elevatedtemperatures, which causes corrosion and degradation of the pyrolysisfurnace and shortens the reactor lifetime. Therefore, methods ofprevention of coke formation are desired in order to improve Vinylchloride production and avoid the coke removal operation.

Great Britain Patent No. 1,494,797, VINYL CHLORIDE BY ADEHYDROCHLORINATION PROCESS, teaches a method of addition of 200-5000PPM of 1,1,2-trichloroethane to reduce coke formation in EDC-VCMpyrolysis furnaces.

U.S. Pat. No. 3,896,182 teaches a method of reducing coke formation andfouling by lowering the oxygen content in the EDC feed.

Coke formation in pyrolysis furnaces continues to be undesirable andthus effective alternative methods to reduce the formation of coke inpyrolysis furnaces are always desired.

SUMMARY OF THE INVENTION

A method of reducing the formation of coke deposits on the heat-transfersurfaces of an ethylene dichloride to vinyl chloride pyrolysis furnacecomprising exposing the heat transfer surfaces of said pyrolysis furnaceto a phosphine selected from the group consisting of phosphines with thegeneral formula:

wherein R₁, R₂ and R₃ are selected from the group consisting ofhydrogen, chlorine, alkyl, aryl, alkylaryl and arylalkyl, wherein R₁, R₂and R₃ can be the same or different.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph illustrating the effectiveness of triphenyl phosphine(TPP) in inhibiting coke formation in comparison with an untreated run.

FIG. 2 is a graph illustrating the effectiveness of tributyl phosphine(TBuP) in inhibiting coke formation in comparison with an untreated run.

FIG. 3 is a graph illustrating the effectiveness of tributyl phosphine(TBUP) in stabilizing VC yield in comparison with an untreated run.

DETAILED DESCRIPTION OF THE INVENTION

The following terms have the indicated meanings:

Coke means the solid carbonaceous material that is an unwanted byproduct of the pyrolysis reaction,

Decoke means the removal of coke from a pyrolysis furnace,

EDC means ethylene dichloride, which is also known as dichloroethane,

EDC-VCM furnaces refers to ethylene dichloride to vinyl chloridepyrolysis furnaces,

PH₃ is known as phosphorus hydride,

TLE refers to transfer line exchanger.

VC means vinyl chloride,

VCM means vinyl chloride monomer.

The instant claimed invention is a method of reducing the formation ofcoke deposits on the heat transfer surfaces of an ethylene dichloride tovinyl chloride pyrolysis furnace comprising exposing the heat transfersurfaces of said pyrolysis furnace to a phosphine selected from thegroup consisting of phosphines with the general formula:

wherein R₁, R₂ and R₃ are selected from the group consisting ofhydrogen, chlorine, alkyl, aryl, alkylaryl and arylalkyl, wherein R₁, R₂and R₃ can be the same or different. For purposes of this patentapplication, “alkyl” means a fully saturated hydrocarbon moiety of fromabout 1 to about 40 carbon atoms. The alkyl moiety may optionally besubstituted with one or more —Cl, —Br, —SO₃, —OR_(a), —SR_(a),—NR_(a)R_(b), —SiR_(a)R_(b)R_(c), and —BR_(a)R_(b) groups, where R_(a),R_(b), and R_(c) are selected from the group consisting of hydrogen,unsubstituted alkyl, unsubstituted aryl, unsubstituted alkylaryl andunsubstituted arylalkyl. The alkyl moiety is connected to the phosphorusatom through a bond to a carbon atom.

For purposes of this patent application, “aryl” means an aromaticmonocyclic or multicyclic ring system radical of about 6 to about 20carbon atoms. Preferably aryl is phenyl or naphthyl or anthracenyl. Thearyl moiety is optionally substituted with one or more alkyl, alkenyl,—Cl, —Br, —SO₃, —OR_(a), —SR_(a), —NR_(a)R_(b), —SiR_(a)R_(b)R_(c), andBR_(a)R_(b) groups, where R_(a), R_(b), and R_(c) are selected from thegroup consisting of hydrogen, unsubstituted alkyl, unsubstituted aryl,unsubstituted alkylaryl and unsubstituted arylalkyl. The aryl moiety isattached to the phosphorous atom by a bond to one of the carbons in thering.

For purposes of this patent application, “alkylaryl” refers to an arylmoiety with at least one alkyl substituent. The alkylaryl moiety isattached to the phosphorus atom by a bond to one of the carbons in thering of the aryl portion of the alkylaryl moiety.

For purposes of this patent application, “arylalkyl” refers to an alkylmoiety with at least one aryl substituent. The arylalkyl moiety isattached to the phosphorus atom by a bond to one of the carbons in thealkyl portion of the arylalkyl moiety.

For purposes of this patent application, “alkenyl” refers to anunsaturated hydrocarbon radical of from about 2 to about 10 carbonatoms. Alkenyl moieties have one double bond. Preferably alkenyl isallyl.

The preferred phosphine compounds are those in which R₁, R₂ and R₃ areall the same.

Suitable phosphine compounds for use in the method of the instantclaimed invention are phosphorous trichloride (PCl₃), triphenylphosphine, tri-n-butyl phosphine, tri-n-octyl phosphine, tridodecylphosphine, tricyclohexyl phosphine, tribenzyl phosphine, triphenylphosphine, tris(p-tolyl)phosphine, tris(1-naphthyl)phosphine,tris(anthracenyl)phosphine, tris(3-chlorophenyl)phosphine andtris(2-methoxyphenyl)phosphine.

The phosphine compounds useful in the method of the instant claimedinvention are either available commercially or can readily besynthesized using techniques known to ordinary people skilled in the artof phosphine compounds.

These phosphine compounds can be applied individually or they can beapplied by mixing one or more of them together.

When being used in the method of the instant claimed invention, thephosphine can be used as neat (pure compound) or it can be blended withsolvents or conversion boosters or a mixture of solvents and conversionboosters. Solvents for phosphines and conversion boosters for the EDC toVCM reaction are known in the art of phosphine chemistry and in the artof EDC to VCM chemistry.

The phosphine can be applied in different ways. The pyrolysis furnacecan be contacted with phosphine prior to ethylene dichloride feed; thisis known as pretreatment. Alternatively, or in conjunction withpretreatment, the pyrolysis furnace may be continuously orintermittently treated with phosphine during processing of EDC feed.This is known as continuous or intermittent treatment, respectively.

For pretreatment, the phosphine can be applied by using any of thefollowing methods, including, but not limited to, spraying, soaking,painting, flushing and chemical vapor deposition. All of these methodsare useful in conducting the method of the instant claimed invention, aslong as they provide an effect contact of the phosphine with thepyrolysis furnace inner surfaces.

One method of pretreatment is to soak the pyrolysis furnace with aphosphine-containing formulation prior to processing EDC feed. Anothermethod of pretreatment is to use a known chemical vapor deposition (CVD)method to lay the phosphine on the inner surfaces of the pyrolysisfurnace.

For continuous or intermittent treatment, the phosphine is added intothe pyrolysis furnace during the presence of EDC feed. The recommendedaddition rate for the phosphine ranges from about 1 to about 5000 partper million (ppm) based on the EDC feed rate by weight, preferably fromabout 10 to about 1000 ppm, and most preferably from about 20 to about200 ppm.

For continuous and intermittent treatment, the phosphine can be injectedat any location prior to the pyrolysis furnace. These injection points,can include, but are not limited to, the inlet to the convection sectionor the crossover section between the convection section and the radiantsection or at the front of the TLE.

The following examples are presented to be illustrative of the presentinvention and to teach one of ordinary skill how to make and use theinvention. These examples are not intended to limit the invention or itsprotection in any way.

EXAMPLES

A bench scale pyrolysis furnace was used to simulate the industrialpyrolysis operation. Also used were a microbalance and a gaschromatograph, allowed the monitoring of coke formation and pyrolysisproduct distribution and conversion.

The bench scale pyrolysis furnace consisted of a coiled preheater(simulating the convection section), a tubular pyrolysis reactor(radiant section), an electronic microbalance and a gas chromatograph(GC). The preheater was made out of Incoloy 800, a Ni—Cr—Fe alloy, andthe pyrolysis reactor was made out of quartz. Both the preheater and thepyrolysis furnace were heated using electrical heaters. A metal alloyspecimen of Incoloy 800H, a Ni—Cr—Fe alloy, was suspended in the radiantsection of the furnace reactor, and its weight was constantly recordedby the microbalance.

During operation of the pyrolysis furnace (cracking EDC) coke formed onthe metal specimen, and thus, any weight increase during the crackingoperation was a measure of coke deposition on the metal coupon. Thetypical output from the microbalance was a plot of coke buildup vs. timeon stream. Two pieces of information from the plot is the total cokeaccumulation during a cracking test and the coking rate at eachindividual moment. The coking rate is a measure of coke accumulation perunit time at a given moment, and it is measured by the slope of the cokebuildup-time curve at that moment.

In addition to the microbalance analysis, the effluent from the exit ofthe pyrolysis furnace was sampled periodically for GC analysis. EDCconversion and VC yield were obtained based on the GC analysis. EDCconversion was defined as the percentage of the EDC feed being consumedthrough the pyrolysis furnace, and the VC yield was the percentage ofthe EDC feed which was converted to VC.

During a cracking operation, EDC feed was pumped into the pyrolysisfurnace at a rate of about 11 cc/hour. The EDC feed was evaporated inthe entrance part of the preheater, and heated to about 400° C. throughthe preheater. The heated EDC was then sent to the pyrolysis reactor forcracking reaction. The pyrolysis temperature was measured at the outsideof the pyrolysis reactor and in the isothermal region of the electricalheater. During a cracking experiment, the temperature was controlled at580° C. at which the EDC conversion was around 65%. The crackingreaction was carried out under atmospheric pressure. The residence timeof the EDC feed in the pyrolysis reactor was estimated around 3 seconds.A typical cracking experiment lasted from about 2 to about 4 hours. Thecoke formation during a cracking experiment was measured by the quantityof the coke formed on the metal specimen and the coking rate after thecoke formation reached a steady state.

The decoke operation was carried out at 550° C. under a continuous flowof a mixture of air, nitrogen and helium. The decoke lasted from 2 to 14hours, and most of the coke was removed during the first hour of decoke.

Cracking experiments were conducted without any phosphine treatment tosecure a “baseline” pattern for coke formation. In the crackingexperiments without any phosphine treatment, it was observed that cokeformation was very low on a fresh metal alloy, and a steady increase incoke formation was seen when the metal alloy specimen went throughcracking-decoking cycles. After a certain number of thecracking-decoking cycles, the coke formation on the metal specimen tooka sudden upturn, indicating a breakdown of intrinsic surface protectionagainst coke formation, and thereafter, the metal specimen maintainedthe high coking activity.

In conducting the method of the instant claimed invention, all the metalalloy specimen were conditioned through numerous cracking-decokingcycles until the high coking activity was obtained, and sustained, andthen, such conditioned metal specimen were used in collecting cokeformation data.

The effectiveness of a phosphine coke inhibitor was measured by how muchreduction in coke formation was obtained when treating a conditionedmetal specimen with the phosphine. In a phosphine treated crackingexperiment, a selected phosphine was premixed in EDC feed at a dosage ofaround 200 ppm, based on the weight of EDC, and the phosphine-containingEDC feed was then used as feed during the cracking experiment.

Table 1 lists the phosphines, which have been tested in the crackingexperiments.

TABLE 1 ADDITIVES LABELS Tributyl phosphine TBuP triphenyl phosphine TPP

FIGS. 1-2 show the coke formation with TPP and TBuP treatment incomparison with an untreated. Clearly, both phosphine compounds areeffective inhibitors for coke formation.

It was found that not all phosphorous containing compounds are effectivein the reduction of coke formation in an EDC-VC pyrolysis furnace.Triphenyl phosphate, triphenyl phosphine oxide, dioctyl phenylphosphonate and ethylhexyl ethylhexyl phosphonate were tested to see ifit they were effective reducers of coke formation in an EDC-VC pyrolysisfurnace. None of these phosphorous containing compounds were found tohave any reduction effect on coke formation.

The effectiveness of the phosphine as coke inhibitor was alsodemonstrated in stabilizing EDC conversion and VC yield. It was observedthat the acceleration in coke formation on a conditioned coupon wasaccompanied by a runaway EDC conversion and a deterioration in VC yield.With a phosphine treatment, both EDC conversion and VC yield held fairlysteady during a cracking operation.

FIG. 3 shows the VC yield obtained during the runs given in FIG. 2(treated with tributyl phosphine in comparison to an untreated run). Asthe figures indicated, for an untreated run, the sharp increase in cokeformation was accompanied by a sharp decline in VC yield, while for atreated run, VC yield held steady during the whole run.

It is believed, without intending to be bound thereby, that thephosphine treatment functions to prevent the breakdown of a surfaceprotective layer on the pyrolysis furnace walls, and with an intactsurface protective layer, the formation of undesired catalytic sites isreduced. With a reduction in undesired catalytic sites, the runaway EDCconversion and deterioration in VC yield is avoided.

The specific examples herein disclosed are to be considered as beingprimarily illustrative. Various changes beyond those described will, nodoubt, occur to those skilled in the art; such changes are to beunderstood as forming a part of this invention insofar as they fallwithin the spirit and scope of the appended claims.

What is claimed is:
 1. A method of reducing the formation of cokedeposits on the heat transfer surfaces of an ethylene dichloride tovinyl chloride pyrolysis furnace comprising exposing said heat transfersurfaces of said pyrolysis furnace to a phosphine selected from thegroup consisting of phosphines with the general formula:

wherein R₁, R₂ and R₃ are selected from the group consisting ofhydrogen, chlorine, alkyl, aryl, alkylaryl and arylalkyl, wherein R₁, R₂and R₃ can be the same or different.
 2. The method of claim 1 in whichR₁, R₂ and R₃ are all the same.
 3. The method of claim 1 in which saidphosphine is selected from the group consisting of phosphoroustrichloride, triphenyl phosphine, tri-n-butyl phosphine, tri-n-octylphosphine, tridodecyl phosphine, tricyclohexyl phosphine, tribenzylphosphine, tris(p-tolyl)phosphine, tris(1-naphthyl)phosphine,tris(anthracenyl)phosphine, tris(3-chlorophenyl)phosphine andtris(2-methoxyphenyl)phosphine.
 4. The method of claim 1 wherein saidphosphine is added continuously or intermittently to a ethylenedichloride feed which is then added to said pyrolysis furnace.
 5. Themethod of claim 4 wherein said ethylene dichloride feed to saidpyrolysis furnace is treated with from about 1 ppm to about 5000 ppm ofsaid phosphine based on the weight of said ethylene dichloride feed. 6.The method of claim 4 wherein said phosphine is added to said pyrolysisfurnace at a location prior to the radiant section of the pyrolysisfurnace.
 7. The method of claim 1 wherein said phosphine is blended withsolvents or conversion boosters and then this blend is added to saidpyrolysis furnace.
 8. The method of claim 1 wherein said heat transfersurfaces are treated with said phosphine prior to processing of ethylenedichloride feed.
 9. The method of claim 8 wherein said heat transfersurfaces are treated with said phosphine for from about 30 minutes toabout two days.
 10. The method of claim 8 wherein said phosphine isapplied to said heat transfer surfaces by a method selected from thegroup consisting of spraying, soaking, painting, flushing and chemicalvapor deposition.